In the conventional systems for pumping of liquefied natural gas (LNG) from one receiver to another, for instance from the tank of a sea-going vessel to a header pipe leading to a shore installation, it is conventional to use a pump which is submerged in the LNG in the vessel's tank. A riser pipe connects the outlet of the pump to a stop valve outside the usual dome of the vessel's tank, and a flexible connection connects the stop valve to a header leading to the shore installation.
With the systems as at present used, there is a possibility that a pocket of LNG in gaseous state becomes formed in the riser pipe between the outlet of the pump and the inlet of the stop valve. When the pump is subsequently operated, the column of liquid advances up the riser pipe and, due to the pressures involved, causes collapse and liquefaction of the gas in the pocket. The column of liquid accordingly slams against the stop valve and can damage the valve or piping.